This invention relates to method and apparatus used for high-temperature silicon processing. More particularly, this invention relates to quartz tubes used with the high-temperature well drive process.
Typically, the well drive process uses the greatest temperatures in the processing of semiconductor wafers. In forming the well areas of a semiconductor circuit on a silicon wafer, the chips are first doped with a dopant material and then annealed at high temperatures so that the dopant material diffuses deeper into the wafer.
Furnaces are used to create the required high temperatures. The wafers are placed in quartz tubes within the furnace. The quartz tube must be kept very clean or any impurities in the chamber tube can damage the semiconductor wafer. The well drive annealing process is currently done at temperatures which are greater than quartz's 1140.degree. C. annealing point. This means that the quartz in the tube will start to slowly flow. Typically, the top of the quartz tube will sag downward. This sagging may eventually cause the quartz to contact the wafers inside the quartz tube. For this reason, the quartz tube needs to be monitored and replaced before the quartz sags so much that it contacts the wafers. Thus, after a period which can be as little as three to six weeks, the quartz tube requires replacement.
When a quartz tube is replaced, 24 to 48 hours of requalifying tests need to be done. These tests include a capacitance/voltage testing, which takes 3 to 4 hours; a hydrogen chloride (HCl) cleansing, which typically takes 8 hours; and an 8 to 16 hour profiling of the thermocouple. Thermocouples are typically placed in each quartz tube to produce an accurate indication of the temperature at the wafers. Due to the precision required for current semiconductor processing, each new thermocouple needs to be calibrated to ensure that the readings of the temperature are accurate. Inaccurate temperature readings by the thermocouple can cause the well areas to be too large or too small and thus throw off the process control.
This 24 to 48 hours of requalifying time every 3 to 6 weeks produces a bottleneck in the wafer fabrication process. The processes that occur after the well drive annealing process (which includes most of the semiconductor processing steps) will be held up until the replaced quartz tube can be requalified.
It is desired that a method and apparatus can be determined which will produce an increased life span for the quartz tubes. This will reduce the expenses involved in replacing the quartz tubes and also reduce the down-time which results from the requalification of a new quartz tube.